The present invention relates to capacitance manometers, and more particularly to low pressure capacitance manometers having a sealed vacuum reference chamber and designed to operate over a pressure scale having an upper limit in the range of 1-1000 Torr.
Early capacitance manometers, such as shown in U.S. Pat. Nos. 2,999,385 and 2,999,386 issued to Russell Wolfe, utilized a capacitance bridge formed by a movable pressure diaphragm and a pair of electrodes, one spaced on either side of the diaphragm. The diaphragm, electrodes and various spacer or housing elements were clamped together with high-pressure metal clips in a manner to form a reference chamber and a pressure measurement chamber, one on either side of the diaphragm. With such a construction, to measure near 0 Torr., the reference chamber required a continuously operating vacuum pump. More recently, with the advent of processes for welding a tensioned metal diaphragm to the housing, it has become possible to provide a permanently evacuated sealed reference chamber, gettered to provide a near-perfect vacuum.
Modern high-vacuum sensors are constructed with a welded housing having a tensioned diaphragm fused along a centerline thereof, with the gettered, sealed reference vacuum chamber on one side of the diaphragm, and a closed chamber with an inlet tube for connection to the pressure system to be measured on the other side. The diaphragm is generally a stainless steel such as Inconel, which may be obtained in sheet form 0.01 mm. thick, more or less, without microscopic pinholes; the housing is preferably of the same or similar material. The electrode structure is mounted within the vacuum chamber, so that the dialectric is constant and the capacitance of the electrode-diaphragm assembly will be a function only of diaphragm displacement, which, in turn, is indicative of the measured pressure. On a sensor designed for a full-scale pressure of 1 Torr. a displacement as small as 1.times.10.sup.-8 cm. can be reliably detected by the change in electrode-diaphragm capacitance caused by the displacement.
Such capacitance sensor units have found widespread commercial use for controlling processes in which the pressure must be monitored, such as in sputtering or other etching or thin film fabrication processes, and these sensors may be purchased together with integrally-mounted electronic circuitry for converting the capacitance of the sensor to a voltage signal directly proportional to the pressure. Such auxiliary electronic circuitry typically contains circuitry which compensates for non-linearities of pressure with displacement, and for temperature effects, and, most importantly, which self-zeroes the output for eaoh run. Because of the delicacy of the diaphragm, and the need to absolutely shield the diaphragm from trauma so that its characteristics remain within the range of the auxiliary circuitry, the sensor units for such systems are made as entirely sealed cans with a single tube opening from the Px side, usually via a baffle or porous filter. However, a problem inherent in such sensors is that when used to monitor a process, contaminants gradually build up on the diaphragm and cause displacements which eventually displace the diaphragm beyond the self-zeroing range of the auxiliary circuitry, or alter the displacement curve beyond the compensating range of the circuitry. The sensor must then be discarded and replaced, after a lifetime of a few weeks, and the circuitry re-adjusted to accommodate the thermal and span characteristics of the new sensor unit.